This invention relates generally to iron-based sintered alloy compositions used for making valve seat inserts for internal combustion engines. Valve seat inserts (VSI) operate in a highly aggressive environment. Valve seat insert alloys require resistance to abrasion and/or adhesion caused by the mating valve seat surface, resistance to the softening and degradation due to the high operating temperatures and resistance to the corrosion induced degradation caused by the combustion products.
Valve seat inserts are machined after insertion in cylinder heads. The cost of machining valve seat inserts is a major contributor to the overall cost of machining cylinder heads. This poses a major problem to valve seat insert alloy design because the hard material phases that endow the alloy with wear resistance also produce severe wear of cutting tools during the machining operations.
Sintered alloys have displaced cast alloys for valve seat insert for most passenger car engine applications. Powder metallurgy (pressing and sintering) is a very attractive VSI manufacturing process because of its alloying flexibility which enables the coexistence of very dissimilar phases such as carbides, soft ferrite or pearlite phases, hard martensite, Cu-rich phase etc., and its near-net shape capability that reduces machining costs.
Sintered valve seat insert alloys have evolved in response to the demands of internal combustion engines-higher power density that results in higher thermal and mechanical loads, alternative fuels for reduced emissions and longer engine life. Those sintered alloys are primarily of four types:
1) 100% tool steel,
2) Pure iron or low-alloy iron matrix with the addition of particles of a hard phase to increase wear resistance,
3) High carbon, high chromium ( greater than 10 wt. %) steels, and
4) Co and Ni base alloys.
These materials have met most of the durability requirements. However, all of them are difficult to machine, in spite of a the use of high percentage of added machinability agents.
Types 1, 2 and 3 are high-carbide-containing materials. U.S. Pat. Nos. 6,139,599, 5,859,376, 6,082,317, 5,895,517 and others describe iron base sintered alloys with hard particles dispersed in a mainly pearlite phase (5 to 100% pearlite) plus isolated fine carbides and self-lubricating compounds for exhaust valve seat applications.
Increasing the amount and size of carbides in the alloy, while increasing durability, is detrimental to processing (compressibility and green strength) and machinability of the finished valve seat insert. In addition, the strength of the sintered product is dramatically reduced by the presence of massive carbides or hard particles.
U.S. Pat. No. 6,139,598 presents a valve seat insert material with a good combination of compressibility, high temperature wear resistance, and machinability. The mixture used to manufacture this material is a complex blend of steel powder containing Cr and Ni ( greater than 20% Cr and  less than 10% Ni), Ni powder, Cu, ferroalloy powder, tool steel powder and solid lubricant powder. While this material may bring significant improvements in compressibility and wear resistance, its high content in alloying elements suggests a high material cost (Ni, Tool steel, Cr rich steel powder, ferroalloys).
U.S. Pat. No. 6,082,317 presents a valve seat insert material in which cobalt-based hard particles are dispersed in a matrix of an iron-based alloy. In comparison with conventional hard particles (carbides), cobalt-based hard particles are claimed to be less abrasive, resulting in reduced wear of the mating valve. It is stated that this material is suitable for applications requiring direct contact between the metallic surfaces of the valve and the valve seat, as used in internal combustion engines. Although Co alloys present a good balance of properties, the price of Co makes these alloys too costly for automotive applications.